Pervious concrete is gaining importance in landscaping and construction applications where reducing run-off and preventing depletion of groundwater supplies are important.
In pervious concrete, carefully controlled amounts of water and cementitious materials are used to create a paste that forms a thick coating around aggregate particles. Unlike regular concrete, however, a pervious concrete mixture contains little or no sand, which results in a substantial void content. Using a carefully controlled quantity of cement paste to coat and bind the aggregate particles together, a system of interconnected voids is created. These interconnected voids create a highly permeable product that allows rain water to pass through quickly. Typically, between 15% and 25% voids by unit volume are achieved in the hardened pervious concrete. Flow rates for water through pervious concrete typically range from 500 to 1,500″/hr. Both the low mortar content and the high porosity also reduce strength compared to regular concrete mixtures, but sufficient strength for many applications can still be readily achieved.
In essence, all concrete is comprised of aggregate such as sand and stones of varying coarseness, cement and water. Pervious concrete (PC), which contains intentionally created interconnected voids throughout its bulk, is also comprised of aggregate, cement and water, but in a unique manner that differentiates it from other concretes.
The ranges of the various components of regular and pervious concrete known in the art are shown in Table 1, where weights are expressed in units per cubic yard of material, and w/c is the water:cement ratio.
TABLE 1coarsehydrationSandaggregatecementstabilizerw/c(lbs)(lbs)(lbs)(oz.)regular0.45-0.601,250-1,8001,250-1,800560-7000pervious0.25-0.40 0-1002,500-2,800400-55016-40
The aggregate in all forms of concrete is suspended in a solution of cement “paste”, which is the combination of water and cement. In “regular” concrete, the particles of aggregate may, but typically do not, make contact with each other, while cement paste completely fills in the regions between the particles of aggregate. In order for the cement paste to effectively fill in the spaces, its viscosity needs to be low enough (i.e., the paste is thin enough) to allow it to flow easily around the individual pieces of aggregate.
When water and cement are mixed together in the right proportions they make a strong, durable, hardened paste. Cement molecules undergo a hydration reaction with water, which results in the growth of calcium silicate hydroxide (“CSH”) crystals that give concrete its strength. If the water/cement ratio is too low, there will not be enough water to fully hydrate the cement, resulting in a weak concrete. If the water/cement ratio is too high, the cement will fully hydrate but the additional volume of water will have spaced the cement molecules father apart, preventing the growing CSH crystals from creating strong bonds with each other.
The general consensus in the scientific community is that the ideal proportion, known as the water/cement (w/c) ratio, for optimum hydration conditions of all concretes is in the range of 0.41-0.43 by weight of the mix, see FIG. 2 (data from Meininger, Concrete International, Vol. 10:8, p. 22 (1988)). Water/cement ratios are typically quoted in the mix, before pouring, curing and setting. W/c ratios in regular concrete however are typically quite a bit higher than the ideal, very often in the range 0.55-0.65, for reasons relating to workability. It is well understood within the engineering and construction fields that, while higher w/c ratios do have a negative impact on concrete strength and performance, the trade-off is none the less important to make the concrete workable in the field. The more water in the mix (up to a ratio of about 0.7), the more readily it flows and the easier it is to discharge from a mixer truck, move into position in the forms, flow around reinforcing bars, and to finish.
On the other hand, in order to maximize the quality and strength of regular concrete it is desirable to avoid using a w/c ratio any higher than is necessary for a given application. To this end a class of chemical admixtures known as water reducers have been developed that allow for a reduction of the w/c ratio to the low 0.50's or even the high 0.40's while maintaining workability.
By contrast, pervious concrete is more like a jar of marbles in that the aggregate particles are packed together closely, and every particle of aggregate makes contact with several other particles of aggregate. There needs to be enough cement paste coating each aggregate particle to bond the aggregate particles to each other where they make contact, and to lock them securely in place, but not so much paste that the spaces between the aggregate particles are entirely filled up. A high level of aggregate to aggregate contact (consolidation) is directly related to the strength of a pervious concrete slab, because it allows for the transfer of loading forces throughout the slab and into the base material. Additionally, pervious concrete tends to be formed from coarse aggregate(s), with little to no fine aggregates in the mix.
Unlike regular concrete, the cement paste viscosity of pervious concrete needs to be high (thick) enough to allow it to cling to the aggregate, but not easily flow, which would cause it to run off the particles of aggregate, e.g., under the force of gravity or from agitation during installation. Such a run off, or drain down, if it occurred, would greatly reduce the bonds between the aggregate, and the accumulation of cement paste in the lower part of the slab would greatly reduce if not eliminate permeability of the overall slab.
As the cement paste in pervious concrete needs to be thick enough to cling to the aggregate and not to flow off, or drain down, in order to achieve that level of cohesion the w/c ratio in pervious concrete needs to be in a relatively narrow range, typically between 0.28-0.32. Above that range, the cement paste becomes too thin and drains off the aggregate. It should be noted that mixes utilizing a w/c ratio as high as 0.40 have been used by incorporating high dosages of viscosity modifiers admixtures (VMA's). VMA's, however, impart considerable stickiness to the cement paste and actually reduce workability to below acceptable levels. Below that range, the cement mixture is not workable and does not form proper bonds between the particles of aggregate. Unfortunately, the typical w/c range is not only well below that required for full hydration, ˜0.41-43, meaning that, in pervious concrete full hydration is not achieved, but the low w/c range is directly responsible for the single most challenging aspect of working with pervious concrete, lack of workability, as well as most pervious concrete failures subsequent to installation.
Pervious concrete is well known for being a thick, relatively dry, stiff product that is difficult to work with and presents challenges at every step of the installation process: batching, transport, placement, finish, and curing. Pervious concrete tends to stick to the inside of the truck drum (as well as to the installation equipment), thereby causing it to discharge slowly from the truck and slide slowly, if at all, down the delivery chute. For this reason, pervious concrete often requires manual assistance, for example in the form of one or two men with shovels, to coax it down the delivery chute. The lack of workability then extends to both the installation and finishing phases where the low w/c pervious concrete requires a considerable degree of effort to move into place, consolidate (compact) and finish.
During installation, PC must be compacted in order to consolidate the aggregate, and create the aggregate to aggregate contact that is critical to load bearing properties. The lower the w/c ratio, the thicker the paste and the more it resists movement during this phase of installation. The increased resistance to flow from the relatively dry, stiff, cement coating on the aggregate increases the amount of compactive effort required during installation, thereby decreasing the degree of consolidation and reducing the critical amount of aggregate to aggregate contact required for strength of the pervious concrete.
The low w/c ratio of PC is also responsible for the next common challenge in working pervious concrete: critical evaporative loss of moisture from the mix during transport and installation.
Because the relatively thin layer of paste coating the aggregates is exposed to the air throughout the entire volume (by contrast with regular concrete where it is only exposed at the exterior surfaces), large amounts of moisture will be lost due to evaporation. From the moment the ready mix truck leaves the batch plant until the plastic curing cover goes on the slab (thereby sealing in the remaining moisture), moisture is being lost due to evaporation. The quantity of moisture loss is sensitive to environmental conditions (temperature, relative humidity, wind speed, sun exposure), and time. The more time it takes to discharge the pervious concrete from the truck, move it into place, consolidate and finish it, the higher the moisture loss will be. Every additional minute needed to discharge and install pervious concrete increases the quantity of water lost to evaporation (thereby lowering the w/c ratio), and subsequently increases the risk of slab failure due to the insufficient hydration that results from such a loss of moisture.
Therefore, typical PC, which starts with a w/c ratio that is already lower than needed for full hydration, and then suffers a further loss of moisture due to the long installation times that result from the thick heavy mix, often gives rise to a weak slab. An insufficiently hydrated PC slab is likely to experience raveling (the contiguous dislodging of aggregate at the surface of the slab) and a compromised in-service performance. For example, from visual observation it is possible to see an average loss of 4-6% w/c ratio during transport and installation. In all but the most ideal conditions, a PC mix designed and batched at the high end of the common range, 0.32 w/c, will actually be a 0.26-0.28 w/c mix by the time the installation is complete; such a reduction in w/c ratio can result in compromised strength and durability of the installed product. Such compromised strength is common in the field.
In sum, in conventional pervious concrete, it is preferred to maintain the w/c ratio at 0.28-0.32. This is to be contrasted with regular concrete, in which the ratio is normally well above 0.5.
Various additives have been utilized to improve workability of PC mixes and durability of the resulting installation. Given the effectiveness of water reducers (WR) to increase workability of regular concrete, it was assumed that they might confer similar benefits on PC. While water reducers do act to thin the paste as one would expect, thinning the paste in PC is not a desirable characteristic for reasons previously stated. To counteract thinning produced by water reducers, a viscosity modifying admixture (VMA) has commonly been used in conjunction with a water reducer. VMA's act to increase the viscosity, or thicken the paste, and are effective in this capacity in a PC mix when used with a water reducer. However, any improvement in workability offered by the water reducer is then offset by the VMA. While the VMA's are effective at offsetting the thinning effects of the WR, they also impart an increased level of stickiness to the pervious concrete mix. PC mixes containing VMA's have proved to be difficult to place and finish because the paste covered aggregate would stick to the truck drum, the discharge chute, the roller, and finishing tools. Making matters worse, water reducers typically have a very limited working life and lose effectiveness after only 30-60 minutes, thereby increasing the thickening and stickiness of the mix. Creating an acceptable finish under these conditions has therefore proven difficult, unless the installation crew applies a release agent liberally and often to the tools and equipment. Since release agents are typically hydrocarbon based, and are strictly regulated in many jurisdictions (and in some instances are not permitted at all), they rarely offset the challenges of installation. In spite of these difficulties, the combination of water reducers, VMA's, and release agents have become common, and equipment manufacturers are now marketing tools specially adapted to work with these types of mixes. By way of example, a finishing cross roller (from Lura Enterprises, West Fargo, N. Dak.) has a built-in brush to remove stuck aggregate from the roller as it turns: (see www.lurascreed.com/previous; scroll down to cross roller with the orange brush).
Another approach to improve workability of PC mixes and durability of the resulting installation is to use a super-absorbent polymer (SAP). SAP's are currently found in personal disposable hygiene products, such as baby diapers and adult protective underwear. SAP's are also commonly used in commercial applications such as for blocking water penetration in underground power or communications cables, as horticultural water retention agents, in control of spill and waste aqueous fluid, and in artificial snow for motion picture and theatrical productions.
In applications of SAP's to pervious concrete, however, studies have focused on improving the internal curing of the concrete in order to obviate use of a plastic curing sheet.
U.S. Patent App. Pub. No. 2010-0285224 describes use of a polypromancic acid based super-absorbent polymer to promote internal curing in order to eliminate a step of coating an area of pervious concrete with a sheet of plastic while curing. The quantities of SAP involved are high (expressed as wt. % of cement), however, in the demonstrated examples: 8-10% by weight (based on a mixture of water, glycogen, slag and the SAP). Additionally, the SAP is accompanied by a dispersant (ground granulated blast furnace slag, amorphous silica and crystalline silica, or Hydromax). Furthermore, there has been little evidence of uptake by the industry (see, for example, the American Society of Civil Engineers (ASCE) manual on Permeable Pavements, 2015, Ed. B. Eisenberg et al., at pages 88-89, which is silent as to such compositions).
Kevern also describes use of a polypromancic acid based SAP to promote internal curing of a pervious concrete (see “Reducing Curing Requirements for Pervious Concrete with a Superabsorbent Polymer for Internal Curing”, Kevern, J. T., and Farney, C., Transportation Research Record: Journal of the Transportation Research Board, No. 2290, Transportation Research Board of the National Academies, Washington, D.C., 2012, pp. 115-121). The proportion of the SAP described by Kevern is 0.375% by weight (1.5-1.8 lbs per cubic yard) but there has also been little evidence of uptake by the industry.
SAPs have also been used to address internal curing and shrinkage in regular concrete, particularly high strength concrete. (See “Application of Superabsorbent Polymers (SAP) in Concrete Construction”, V. Mechtcherine, and H.-W. Reinhardt. Eds., Springer, 2011, at pages 88, 92, and 114.) The lowest reported proportions of the SAP (in that case, acrylic acid-co-acrylamide) are 0.3 wt. %, and the range extends to 0.6 wt. %.
Accordingly, there is a need for a method of improving a PC composition to make it more easily workable without sacrificing quality of the finished product.
The discussion of the background herein is included to explain the context of the technology. This is not to be taken as an admission that any of the material referred to was published, known, or part of the common general knowledge as at the priority date of any of the claims found appended hereto.
Throughout the description and claims of the instant application the word “comprise” and variations thereof, such as “comprising” and “comprises”, is not intended to exclude other additives, components, integers or steps.